> I imagine that the top 100 of anything (that a lot of people do) that brings in income or fame will still be there in 5 years.

This sort of blanket assumption is exactly what the parent is arguing against. The mortality rate of top-n things is relatively easy to measure, and should be baselined first. Then we can compare recent performance vs historical performance, and actually say if something has changed. There's no need to start with the assumption "not much changes over 5 years" -- it can be measured instead.